Displays may be broadly classified into one of two types: emissive (e.g., CRTs and plasma display panels (PDPs)) or non-emissive. This latter family, to which liquid crystal displays (LCDs) belong, relies upon an external light source, with the display only serving as a light modulator. In the case of liquid crystal displays, this external light source can be either ambient light (used in reflective displays) or a dedicated light source (such as found in direct view displays).
Liquid crystal displays rely upon three inherent features of the liquid crystal (LC) material to modulate light. The first is the ability of the LC to cause the optical rotation of polarized light. Second is the ability of the LC to establish this rotation by mechanical orientation of the liquid crystal. The third feature is the ability of the liquid crystal to overwrite this mechanical orientation by the application of an external electric field.
In the construction of a simple, twisted nematic (TN) liquid crystal display, two substrates surround a layer of liquid crystal material. In a display type known as Normally White, the application of alignment layers on the inner surface of the substrates creates a 90° spiral of the liquid crystal director. This means that the polarization of linearly polarized light entering one face of the liquid crystal cell will be rotated 90° by the liquid crystal material. Polarization films, oriented 90° to each other, are placed on the outer surfaces of the substrates.
Light, upon entering the first polarization film, becomes linearly polarized. Traversing the liquid crystal cell, the polarization of this light is rotated 90° and is allowed to exit through the second polarization film. Application of an electric field across the liquid crystal layer aligns the liquid crystal directors with the field, interrupting its ability to rotate light. Linearly polarized light passing through this cell does not have its polarization rotated and hence is blocked by the second polarization film. Thus, in the simplest sense, the liquid crystal material becomes a light valve, whose ability to allow or block light transmission is controlled by the application of an electric field.
The above description pertains to the operation of a single pixel in a liquid crystal display. High information type displays require the assembly of a few million of these pixels, which are referred to as sub pixels, into a matrix format. Addressing, or applying an electric field to, all of these sub pixels while maximizing addressing speed and minimizing cross-talk presents several challenges. One of the preferred ways to address sub pixels is by controlling the electric field with a thin film transistor located at each sub pixel, which forms the basis of active matrix liquid crystal display devices (AMLCDs).
The manufacturing of these displays is extremely complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled. The downdraw sheet or fusion process, described in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat. No. 3,682,609 (Dockerty), is one of the few capable of delivering such product without requiring costly post forming finishing operations, such as lapping and polishing.
Typically, the two substrates that comprise the display are manufactured separately. One, the color filter plate, has a series of red, blue, and green organic dyes deposited on it. Each of these primary colors must correspond precisely with the pixel electrode area of the companion, active, plate. To remove the influence of differences between the ambient thermal conditions encountered during the manufacture of the two plates, it is desirable to use glass substrates whose dimensions are independent of thermal condition (i.e., glasses with lower coefficients of thermal expansion). However, this property needs to be balanced by the generation of stresses between deposited films and the substrates that arise due to expansion mismatch. It is estimated that an optimal coefficient of thermal expansion is in the range of 28-33×10−7/° C.
The active plate, so called because it contains the active, thin film transistors, is manufactured using typical semiconductor type processes. These include sputtering, CVD, photolithography, and etching. It is highly desirable that the glass be unchanged during these processes. Thus, the glass needs to demonstrate both thermal and chemical stability.
Thermal stability (also known as thermal compaction or shrinkage) is dependent upon both the inherent viscous nature of a particular glass composition (as indicated by its strain point) and the thermal history of the glass sheet as determined by the manufacturing process. U.S. Pat. No. 5,374,595 discloses that glass with a strain point in excess of 650° C. and with the thermal history of the fusion process will have acceptable thermal stability for active plates based both on a-Si thin film transistors (TFTs) and super low temperature p-Si TFTs. Higher temperature processing (such as required by low temperature p-Si TFTs) may require the addition of an annealing step to the glass substrate to ensure thermal stability.
Chemical stability implies a resistance to attack of the various etchant solutions used in the manufacture processes. Of particular interest is a resistance to attack from the dry etching conditions used to etch the silicon layer. To benchmark the dry etch conditions, a substrate sample is exposed to an etchant solution known as 110 BHF. This test comprises immersing a sample of glass in a solution of 1 volume of 50 wt. % HF and 10 volumes 40 wt. % NH4F at 30° C. for 5 minutes. The sample is graded on weight loss and appearance.
In addition to these requirements, display manufacturers also require substrates with extremely smooth surfaces. Currently, the fusion process offers the potential of a “fire-polished” surface, which is smooth to an atomistic level (Ra of about 0.3 nm as measured by AFM over a 20 micron area). However, subsequent processing steps, such as sheet separation, packaging, etc, can degrade the surface condition through the presence of particles and/or surface damage. To protect against such extrinsic damage, substrate glass can be coated with various polymer films to protect the glass surface during shipment between the substrate manufacture and display manufacture. Also, coating the substrate can enable substrates to be densely packed together in the final shipping configuration.
Current methods for coating substrate glass with a protective polymer film suffer from a couple of shortcomings. First, the polymer coating must be applied to the glass cold to prevent staining of the glass. This means that the coating is not present to protect the surface against particle contamination arising from these two steps. Second, since the polymer coating is not transparent, the coating must be removed from the glass after finishing to allow for the final quality inspection processes. What is needed in the art are substrate glasses with properties suitable for use in displays, and methods and compositions for protecting the surfaces of such substrates during substrate manufacture, shipping, and beyond. The compositions and methods disclosed herein meet these and other needs.